Conversion of hydrocarbons



Patented May 9, 1939 v PATENT OFFICE CONVERSION OF HYDROCARBONS JacqneC. Morrell and Aristid V. Grosse, Chicago,

11]., assignors to Universal Oil Products Company, Chicago, III., acorporation of Delaware No Drawing.

Application September 30, 1937,

Serial No. 166,583

4 Claims.

This invention relates particularly to the conversion of straight chainhydrocarbons into closed chain or cyclic hydrocarbons.

More specifically it is concerned with a process 5 involving the use ofspecial catalysts and specific conditions of operation in regard totemperature, pressure and time of reaction whereby aliphatichydrocarbons can be efiiciently converted into aromatic hydrocarbons.

In the straight pyrolysis of pure hydrocarbons or hydrocarbon mixturessuch as those encountered in fractions from petroleum or other naturallyoccurring or synthetically produced hydrocarbon mixtures the reactionsinvolved which produce aromatics from parafiins and olefins are of anexceedingly complicated character and cannot be very readily controlled.

It is generally recognized in the thermal decomposition of hydrocarboncompounds or by- 20. drocarbon mixtures of relatively narrow range that,whatever intermediate reactions are involved, there is an overall lossof hydrogen, a

tendency to carbon separation and a generally Wider boiling range in thetotal liquid products as 5 compared with the original charge. Under mildcracking conditions involving relatively low temperatures and pressuresand short times of exposure to cracking conditions it is possible tosome extent to control cracking reactions so that they are limited toprimary decompositions and. there is a minimum loss of hydrogen and amaximum production of low boiling fractions consisting of compoundsrepresenting the fragments of the original high molecular weightcompounds. 1.) As the conditions of pyrolysis are increased in severityusing higher temperatures and higher times of exposure to pyrolysisconditions, there is a progressive increase in loss of hydrogen and alarge amount of secondary reactions involving recombination of primaryradicals to form polymers and some cyclization to form naphthenes andaromatics, but the mechanisms involved in these cases are of socomplicated a nature that very little positive information has beenevolved in spite of the large number of theories proposed. In general,however, it may be said that starting with paraffin hydrocarbonsrepresenting the highest degree of saturation that these compounds arechanged progressively into olefins, naphthenes, aromatics, and finallyinto carbon and hydrogen and otherlight fixed gases. It is not intendedto infer from this statement that any particular success has attendedthe conversion of any given paraflin or other aliphatic hydrocarbon 55into an aromatic hydrocarbon of the same number of carbon atoms by wayof the progressive steps shown. If this is done it is usually with verylow yields which are of very little practical significance.

The search for catalysts to specifically control and accelerate desiredconversion reactions among hydrocarbons has been attended with the usualdifliculties encountered in finding catalysts for other types ofreactions since there are no basic laws or rules for predicting theeffectiveness of catalytic materials and the art as a. whole is in amore or less empirical state. In using catalysts even in connection withconversion reactions among pure hydrocarbons and particularly inconnection with the conversion of the relatively heavy distillates andresidue which are available for cracking, there is a general tendencyfor the decomposition reactions to proceed at a very rapid rate,necessitating the use of extremely short time factors and very accuratecontrol of temperature and pressure to avoid too extensivedecomposition. There are further difliculties encountered in maintainingthe efiiciency of catalysts employed in pyrolysis since there is usuallya rapid deposition of carbonaceous materials on their surfaces and intheir pores.

The foregoing brief review of the art of hydrocarbon pyrolysis is givento furnish a general background for indicating the improvement in suchprocesses which is embodied in the present invention, which may beapplied to the treatment of pure parafiin or olefin hydrocarbons,hydrocarbon mixtures containing substantial percentages of paraffinhydrocarbons such as relatively close out fractions producible bydistilling petroleum, and analogous fractions which contain unsaturatedas well as saturated straight chain hydrocarbons, such fractionsresulting from cracking operations upon the heavier fractions ofpetroleum.

In one specific embodiment the present invention comprises theconversion of aliphatic hydrocarbons including paraflin and olefinhydrocarbons into aromatic hydrocarbons by subjecting them at elevatedtemperatures of the order of 400-700 C. to contact for definite times ofthe order of l to seconds with catalytic materials comprising majorproportions of relatively inert carriers and minor proportions of oxidesof vanadium and chromium.

In a still more specific and preferred embodiment the catalysts compriseactivated aluminum oxide which has received successive and alternatedeposits of the two types of oxides. For example, a portion of thevanadium oxide desired in the final composite may first be deposited,then a portion or all of the desired quantity of chromium oxides, afterwhich the remaining amount of vanadium oxides is deposited. Thisprocedure may be modified so that chromium oxides are first deposited,then vanadium oxides and lastly further amounts of chromium oxide or anyamount or method of alternation may be employed. The advantages in thismethod of preparation are definite although the reasons are not fullyunderstood.

According to the present invention aliphatic or straight chainhydrocarbons having six or more carbon atoms in chain arrangement intheir structure are cataiytically dehydrogenated in such a way that thechain of carbon atoms undergoes ring closure with the production in thesimplest case of benzene from n-hexane or n-hexene and in the case ofhigher molecular weight parafllns of various alkyl derivatives ofbenzene. Under properly controlled conditions of times of contact,temperature and pressure very high yields of the order of '75 to 90% ofthe benzene or aromatic compounds are obtainable which are far in excessof any previously obtained in the art either with or without catalysts.For the sake of illustrating and exemplifying the types of hydrocarbonconversion reactions which are specifically accelerated under thepreferred conditions by 'the present types of catalysts, the followingstructural equations are introduced.

on, on O! Om oh on n. on. n (m Q C a C n-Hexanc benzene on, om-om on cnIn the foregoing table the structural formulas of the primary parafllnhydrocarbons have been represented as a nearly closed ring instead of bythe usual linear arrangement for the sake of indicating the possiblemechanisms involved. No attempt has been made to indicate the possibleintermediate existence of mono-oleflns, diolefins, hexamethylenes oralkylated hexamethylenes which might result from the loss of variousamounts of hydrogen. It is not known at the present time whether ringclosure occurs at the loss of one hydrogen molecule or whetherdehydrogenation of the chain carbons occurs so that the first ringcompound formed is an aromatic such as benzene or one of itsderivatives. The above three equations are of a relatively simplecharacter indicating generally the type of reactions involved but in thecase of n-paraillns or mono-oleflns of higher molecular weight than theoctane shown and in the case of branched chain compounds which containvarious alkyl substituent groups in different positions along thesix-carbon atom chain, more complicated reactions will be involved. Forexample, in the case of such a primary compound as 2,3-dimethyl hexanethe principal resultant product is apparently o-xylene although thereare concurrently produced definite yields of such compounds as ethylbenzene indicating an isomerization of two substituent methyl groups. Inthe case of nonanes which are represented by the compound2,3,4-trimethyl hexane, there is formation not only of mesitylene butalso of such compounds as methyl ethyl benzol and various propylbenzols.

It will be seen fronf the foregoing that the scope of the presentinvention is preferably limited to the treatment of aliphatichydrocarbons which contain at least 6 carbon atoms in straight chainarrangement. In the case of paraflin hydrocarbons containing less than 6carbon atoms in linear arrangement, some formation of aromatics may takeplace due to primary isomerization reactions although obviously theyextent of these will vary considerably with the type of compound and theconditions of operation. The

process is readily applicable to paraflins from hexane up to dodecaneand their corresponding oleflns. With increase in molecular weightbeyond this point the percentage of undesirable side reactions tends toincrease and yields of the desired alkylated aromatics decrease inpropor tion. Beginning with decane and under the more severe conditionsof operation with regard to temperature and time of contact with thecatalyst, there is some tendency for the formation of polynuclear cycliccompounds such as naphthalene and anthracene which result from tooextensive hydrogenation reactions. However, when conditions are properlychosen, the formation of these compounds may be kept at a practicalminimum.

The present invention is characterized by the use of a particular groupof composite catalytic materials which employ as their base catalysts orsupporting materials certain refractory oxides and silicates which inthemselves may have some slight specific catalytic ability in thedehydrogenation and cycllzation reactions but which are improved greatlyin this respect by the addition of vanadium and chromium oxides in minorproportions. The base supporting materials are preferably of a ruggedand refractory character capable of withstanding the severe use to whichthe catalysts are put in regard to temperature during service and inregeneration by means of air or other oxidizing gas mixtures after theyhave become fouled with carbonaceous deposits after a period of service.As examples of materials which may be employed in granular form assupports for the preferred catalytic substances may be mentioned thefollowing:

Magnesium oxide Montmorillonite clays Aluminum oxide Kieselguhr BauxiteCrushed flrebrick Bentonite clays Crushed silica Glauconite (greensand)of elements or compounds have been found to be more or less equivalentin accelerating certain types of reactions.

In regard to the base catalytic materials which are employed as supportsaccording to the present invention, some precautions are necessary toinsure that they possess proper physical and chemical characteristicsbefore they receive the successive depositions of vanadium and chromiumoxides. In regard to magnesium oxide, which may bealternativelyemployed, this is most conveniently prepared by the calcination of themineral magnesite. This mineral is of quite common occurrence andreadily obtainable in quantity at a reasonable figure. The pure compoundbegins to decompose to form the oxide at a temperature of 350 0., thoughthe rate of decomposition only reaches a practical value at considerablyhigher temperatures, usually of the order of 800 C. to 900 C. Magnesiumcarbonate prepared by precipitation or other chemical methods may beused alternatively in place of the natural mineral. It is not necessarythat the carbonate be completely converted to oxide but as a rule it ispreferable that the conversion be at least over 90%, that is, so thatthere is less than 10% of the carbonate remaining in the ignitedmaterial.

A very effective catalyst consists of activated aluminum oxide(preferably slightly alkalized) supporting successive depositions ofabout 2% vanadium sesqu'ioxide V203, 4% of chromium sesquioxide CrzOsand another 2% of the vanadium compound. The exact method of preparationof this type of catalyst will be described in later paragraphs ingreater detail. It is essential to the preparation of these catalyststhat the aluminum oxide possess certain structural characteristicspermitting the maintenance of a stable deposit of the oxides on itssurface which is I essentially undisturbed under the conditions ofoperation and when regenerating by burning oif carbonaceous depositswith air or other oxygencontaining gas mixtures. Aluminum oxide which isgenerally preferable as a base material for the manufacture of catalystsfor the process may be obtained from some natural aluminum oxideminerals or ores such as bauxite or carbonates such as Dawsonite byproper calcination, or it may be prepared by precipitation of aluminumhydroxide from solutions of aluminum sulfate, nitrate, chloride, ordifierent other salts, and dehydration of the precipitate of aluminumhydroxide by heat. Usually it is desirable and advantageous to furthertreat it with air or other gases, or by other means to activate it priorto use.

Two hydrated oxides of aluminum occur in nature, to wit, bauxite havingthe formula A1203.2H2O and diaspore having the formula A12O3.H2O. Ofthese two minerals only the corresponding oxide from the bauxite issuitable for the manufacture of the present type of catalysts and thismaterial has generally given the best results of any of the supportingmaterials which have been tried. The mineral Dawsonite having theformula NaaAl(C0a)a.2Al(OH)s is another mineral which may be used as asource of al uminum oxide, the calcination of this mineral giving analkalized aluminum oxide which is apparently more effective as a supportin that the catalyst is more readily regenerated after a period ofservice. Alumina in the form of'powdered corundum is not suitable as asupport.

It is best practice in the final steps of preparing aluminum oxide as abase catalyst to ignite it for some time at temperatures within theapproximate range of from GOO- 700 C. This does not correspond tocomplete dehydration of the oxide but gives a catalytic material of goodstrength and porosity so that it is able to resist for a long period oftime the deteriorating effects of the service and reactivation periodsto which it is subjected.

The element chromium has several oxides, the four best known being 0x0,C1203, C1304, and CrOa. The sesquioxide Cl'zO: is readily produced byheating salts of chromium or the trioxide in hydrogen or hydrocarbonvapors at temperatures above 300 C. The dioxide has been considered tobe an equimolecular mixture of the trioxide and the sesquioxide. Theoxides arereadily dedeveloped on the surfaces and pores of aluminagranules by utilizing primary solutions of chromic acid HaClOs orchromium nitrate Cr(NOa) a. The igmtion of the chromic acid, the nitrateor a precipitated trihydroxide produces primarily the trioxide which isthen reduced to the sesqui oxide to furnish one of the catalytic oxides;

The oxide of vanadium which results from the ignition of the nitrate,the hydroxide or the carbonate is principally the pentoxide V205 whichis reduced by hydrogen at a red heat to form the tetroxide V20: orthecorresponding dioxide V0: and then to the sesquioxide V203. Theproduction of the deposits of sesquioxide upon the granular carriers(preferably alumina) may be made by the use of vanadyl nitrate orsolutions of aluminum or alkali metal vanadates, some of which furnishalkaline residues on ignition. The use of ammonium vanadate ispreferred.

.The following example of the preparation of the catalyst comprisedwithin the scope of the invention and its use in dehydrocyclizationreactions is given to show the advantages and practicality of theinvention although not with the intent of imposing undue limitationsthereon.

A granular activated alumina comprising particles of approximately 4-12mesh was given successive additions of vanadium sesquioxide, chromiumsesquioxide, and a further amount of the vanadium compound by firstadding a solution of ammonium vanadate of a concentration and in anamount suiiicient to leave a residue of about 2% V20: by weight afterreduction of the primary oxides. After drying, a solution of chromiumnitrate was then added to produce ultimately a deposit of 4% by weightof CrzOa after which the primary step of adding the ammonium vanadatesolution was repeated so that ultimately after drying and reducing thecomposite catalyst had the following approximate compomtion:

Per cent Alone 92 Van! 4 CriQ 4 Once-through Catalyst yield of toluenePercent 92; AhOs-S a Ciao: 02 0 51103-8 a V10; 21 92% Altos-4 g Chou-4%Viol 30 It will be seen from the above data that the activity of thecatalyst comprising 92% alumina and supporting 4% each of chromium andvanadium sesquioxides was approximately greater than that of thecatalysts comprising respectively 92% aluminum oxide and 8% of eitherchromium sesquioxide or vanadium sesquioxide. It was further observedthat the rate of carbon formation due to side reactions was much less inthe case of the mixed oxide catalyst as compared with the two simpleroxide catalysts. This indicates that there is evidently a doublepromotion efiect so that there is a definite advantage gained in usingboth oxides.

The nature of the present invention as an improvement in catalytichydrocarbon conversion reactions is evident from the precedingspecification and numerical data although neither section is intended tobe unduly limiting thereon.

We claim as our invention:

1. A process for the production of aromatic hydrocarbons from aliphatichydrocarbons of from six to twelve carbon atoms, which comprisesdehydrogenating and cyclicizing the aliphatic hydrocarbon by subjectionto a temperature of the order of 400 to 700 C. for a period of time notexceeding 10 seconds in the presence of a catalyst comprising vanadiumoxide and chromium oxide.

2. A process for the production of aromatic hydrocarbons from aliphatichydrocarbons of from six to twelve carbon atoms, which comprisesdehydrogenating and cyclicizing the aliphatic hydrocarbon by subjectionto a temperature of the order of 400 to 700 C. for a period of time notexceeding 10 seconds in the presence of a catalyst comprising a majorproportion of aluminum oxide and minor proportions of vanadium oxide andchromium oxide.

3. A process for the production of aromatic hydrocarbons from aliphatichydrocarbons of from six to twelve carbon atoms, which comprisesdehydrogenating and cyclicizing the aliphatic hydrocarbon by subjectionto a temperature of the order of 400 to 700 C. for a period of time ofabout 1 to 10 seconds in the presence of a catalyst comprising vanadiumsesquioxide and chromium sesquioxide.

4. A process for the production of aromatic hydrocarbons from aliphatichydrocarbons of from six to twelve carbon atoms, which comprisesdehydrogenatingl and cyclicizing the aliphatic hydrocarbon by subjectionto a. temperature of the order of 400 to 700 C. for a period of time ofabout 1 to 10 seconds in the presence of a catalyst comprising a majorproportion of activated aluminum oxide and minor proportions of vanadiumsesquioxide and chromium sesquioxide.

JACQUE C. MORRELL. ARISTID V. GROSSE.

